In a typical bulk CMOS process, the bulk of a transistor diode is grounded. Therefore, the threshold voltage of the transistor diode in a typical bulk CMOS process is high due to the back-gate effect. As a result, the threshold voltage of each transistor diode stage increases as the source to bulk voltage, the voltage across source to bulk (i.e., VSB), of the transistor diode for each stage increases.
Moreover, in the typical bulk CMOS process, as the threshold voltage of each transistor diode stage increases, a lower maximum output voltage and lower voltage conversion efficiency results.